A magnetron produces a microwave output signal, the frequency of which is primarily dependent on the frequency characteristics of a resonant cavity associated with the magnetron. In a number of applications it is desirable that the frequency of oscillation can be adjusted or tuned. Previously, tuning of the oscillation frequency has been achieved by including a tuner connected to relatively complex actuator mechanisms such as mechanical linkages and bellows. Such an arrangement is difficult to fabricate and is thus expensive.